GSM (Global System for Mobile communication) is the very popular second generation digital cellular mobile communication standard defined by the European Telecommunications Standards Institute. FIG. 1 is a schematic diagram showing a configuration of a conventional GSM system 10. The GSM system 10 includes Mobile terminals (MS) 12, Base Station Subsystems (BSS) 14, each composed of a plurality of Base Transceiver Stations (BTS) 16 and a Base Station Controller (BSC) 18, and a Network and Switch Subsystem (NSS) 22 composed of a plurality of Mobile Switching Centers (MSC) 20, Home Location Registers (HLR) 30, Visitor Location Registers (VLR) 32 and Authentication Centers (AuC) 34. The Mobile terminal (MS) 12 denotes physical equipment, such as a car phone or other portable phone, used by mobile subscribers to communicate with other mobile subscribers within the subscribed network or with users outside the subscribed network, such as users within a Public Switched Telephone Network (PSTN) 24. The Mobile Switching Centers (MSC) 20, utilized to switch communicating connections, are communicable with a Public Switched Telephone Network (PSTN) 24 and with at least one Base Station Controller (BSC) 16. The Base Station Controller (BSC) 18 is utilized to handover radio connections, and the Base Transceiver Station (BTS) 16 includes physical equipment, such as a radio tower, for transmitting and receiving radio signals. The Home Location Registers (HLR) 30 is a database maintaining all subscriber information, such as user profiles, current location information, International Mobile Subscriber Identity (IMSI) numbers and other administrative information. The Visitor Location Registers (VLR) 32 is a database containing location information about all of the Mobile terminals (MS) 12. The Authentication Center (AuC) 34 is connected to the Home Location Register (HLR) 30 and provides it with authentication parameters and ciphering keys utilized for security purposes.
With reference to FIG. 2, a GSM Public Land Mobile Network (PLMN), designated by reference numeral 200, is shown, which is composed of a plurality of areas 220, each comprising a Mobile Switching Center (MSC) 240 and an integrated Visitor Location Register (VLR) 260. MSC/VLR areas 220 include a plurality of Location Areas (LA) 280, which are defined as that part of a given MSC/VLR area 220 in which a Mobile terminal (MS) 300 may move freely without having to send update location information to the MSC/VLR area 220 that controls the Location Area (LA) 280. Each Location Area 280 is divided into a number of cells 320, for example one to three cells. It should be noted that the Base Station controller (BSC) 250 may be connected to several Base Transceiver Stations (BTS) 270, and may be implemented as a stand-alone node or integrated with the Mobile Switching Center (MSC) 240. In either case, the Base Station Controller (BSC) 250 and the Base Transceiver Station (BTS) 270, as a whole, are generally referred to as Base Station Subsystems (BSS) 320.
A GSM system consists of a plurality of Base Station Subsystems (BSS), and each Base Station Subsystem (BSS) is composed of several cells having their specific coverage area related to the physical location and the antenna direction of the Base Station Subsystems (BSS). When a Mobile terminal (MS) is making a phone call or sending a short message, it must locate in the coverage area of one cell. By mapping the cell database and Cell ID, the area where the Mobile terminal (MS) is located is known in a process called Cell Global Identity (CGI). A CGI is a sub-unit of a location area and defines the particular cell within which the Mobile terminal (MS) is located. Each cell includes Network Name, MSC, BSC number, Site ID, Type (Macro type, Micro Type, Indoor Cell, Outdoor Cell), location (longitude, latitude), Cell number, antenna direction, type, height and Excess Information Rate of the antenna.
Typical GSM systems currently utilize Time Division Multiple Access (TDMA) to handle radio traffic in each cell such that each frequency is shared by eight users. The length of a GSM time frame is 4.615 ms, and is divided into eight time slots by TDMA techniques. However, in other systems utilizing TDMA, more or fewer time slots may by used. The uplink time frame occurs later than the downlink time frame by three time slots, so as to prevent the Mobile terminal (MS) from transmitting and receiving signals simultaneously. Since the distance between each Mobile terminal (MS) and the serving Base Station Subsystems (BSS), as well as the transmission time for radio signals, is not constant, it is necessary to provide specific devices to effect synchronization; that is, the Timing Advance (TA) value must be adjusted to ensure that there is the three time slots difference between uplink and downlink frames. The Timing Advance (TA) value is calculated in accordance with the uplink signals received by Mobile terminals (MS) and is reported approximately twice every second.
For a GSM system, a GMSK modulation is used with a data rate of 270 Kbits per second, therefore, each bit occupies a pulse with a length of approximately 1100 m (c/(270 Kbits/sec), where c is light speed). Since time synchronous signals are transmitted from the Mobile terminal to the Base Transceiver Station, and then round back from the Base Transceiver Station to be received by the Mobile terminal, a resolution of approximately 550 m (1100 m÷2) is achieved. The Timing Advance (TA) value is a number ranging from 0-63, with each number corresponding to approximately a 550 meter radial distance from a receiving Base Transceiver Station (BTS).
Referring to FIGS. 3a and 3b, in general, from the viewpoint of directivities of antennas, the base stations can be classified into two types. One is equipped with an omnidirectional antenna 301 as illustrated in FIG. 3a, where the antenna pattern 310 is omnidirectional, and thus, the base station acquires no information about the aspect of the mobile terminal. In general, this type of base station is installed in relatively lower traffic areas. The other type of Base Station system is typically used in a high traffic area, with the base station being equipped with several directional antennas, one of which is shown in FIG. 3b as 302, whereby each antenna pattern is concentrated for narrow coverage, and different antennas point in different orientations. Thus, each antenna controls a sector region as shown at 320. For example, if there are three antennas (not shown) mounted on a Base Station subsystem, then the main lobe of each antenna is directed to a different direction so that the lobes of the three antennas are spaced equally with an angle difference of 120 degrees. In general, since only signals from those mobile phones within the coverage of the antenna are received by the antenna, each antenna controls a sector region with an angle coverage of 120 degrees.
Once a Timing Advance (TA) value is determined for one Base Transceiver Station (BTS), the distance between the MS and that particular Base Transceiver Station (BTS) is known, but the actual location is not. If the Timing Advance (TA) value equals zero, the Mobile terminal (MS) could be anywhere in a circular region of radius of 550 meters. If the Timing Advance (TA) value equals one, the Mobile terminal (MS) could be anywhere in an annular region from a radius of 550 meters to a radius of 1100 meters. Even for a Base Station Subsystems (BSS) composed of three sector cells which cover a fan-shaped 120 degrees respectively, when the Timing Advance (TA) value equals zero, the Mobile terminal (MS) is located in an sector area of radius of 550 meters; when the Timing Advance (TA) value equals one, the Mobile terminal (MS) is located in a sector area of inner radius of 550 meters and an outer radius of 1100 meters.
A prior art searching method is disclosed by Ericsson (ETSI TS 100 912 V8.6 (2000-11), Technical Specification, Digital cellular telecommunication system (phase 2+), radio subsystem synchronization (3GPP TS 05.10 version 8.6.1 Telease 1999) where the searching regions are initially classified according to the searching area. Referring to FIG. 4, for a Base Station Subsystem utilizing an omnidirectional antenna, since no directivity information can be provided (i.e., the Base Station Subsystem has no information about the aspect of the mobile terminal), the searching region is an annular area 402 having a radius determined by the Timing Advance (TA). For the first searching region with TA=0, the searching region is a round section 401 with a radius of 550 m and thus the region has an area of about 0.95 km2. For the second searching region with TA=1, the searching region is an annular section 402 with an inner radius of 550 m and an outer radius of 1100 m and thus has a searching area of about 2.85 km2, being three times greater than searching region 401. Because of this large area, it is unrealistic, especially for a pedestrian, to search for an object in this larger area. In fact, each time the TA is increased by one, the searching area will become ((TA+1)2−1) times of the first search region; i.e., the third searching region with TA=2 will have an area eight times the first region. Thus, the area increases rapidly so that it becomes unfeasible for a user to search for an object in such a large area.
For those Base Station Subsystems utilizing directional antennas, as stated above, each antenna controls a sector section. Assuming that there are three antennas in the Base Station System, each antenna controls a sector region with an angle coverage of 120 degrees. For the case of a Base Station Subsystem with directional antennas, to derive the search regions, it is necessary to firstly find the sector region and then determine a search region from this sector region. In this case, the sector region has a radius determined by the TA. For the first sector region with TA=0, the region has a sector shape having a radius of 550 m and thus, the region has an area of about 0.32 km2. For the second sector region with TA=1, the region is a sector with an inner diameter of 550 m and an outer diameter of 1100 m (as illustrated in FIG. 4), thus having an area of about 0.95 km2, which is three times the former area. For the third searching region with TA=2, the region is a sector with an inner diameter of 1100 m and an outer diameter of 1650 m and thus has an area of about 2.85 km2, being eight times the first area. Again, this is a large area. In fact, each time TA is increased by one, the area becomes ((TA+1)2−1) times the first search region; i.e., the third sector region with TA=2 will have an area eight times the first region, and for TA=3, the area of the sector becomes 15 times the second region, and so on.
Ericsson's method to define the search region for cells utilizing directional antennas utilizes a circular region with a center being at the middle point of the line forming the maximum diameter of the region. Therefore, for the first search region with TA=0, the area thereof is 2.25 times (i.e., [((3/)1/2A/2)2π]/[(⅓)πA2]) the sector region with TA=0. For TA=1, the ratio of the search region to the sector region becomes larger. Typically, a telecommunications company provides information regarding the locations of restaurants, drug stores, gasoline stations, and the like, within the search region to the mobile terminal in the corresponding sector region. However, the conventional system's search region is too large for most purposes. For example, if TA=2, the search region will be nearly 3 km2, a too large area for a pedestrian to search for a particular object.
The problem with conventional systems such as Ericsson's is that the scale of 550 m is too large to be generally useful. Such a large 550 meters scale utilized in Mobile terminal (MS) location determination is clearly inadequate and is very impractical for mobile subscribers in an urban area for specific services, particularly urgent needs such as emergency aid. Moreover, neighboring cells and directions can not be handled. Therefore, there is a need for a new method and system to define search regions providing higher resolution so that a telecommunication system can provide useful information to mobile subscribers.